Traveling-wave tube



Sep 23, 1958 L. M. FIELD 3 6 TRAVELING-WAVE TUBE Filed July 30, 1956 IIII ll INVENTOR. 4557-59 M 774-20 firrolewz-ys United States Patent MTRAVELING-WAVE TUBE Lester M. Field, Los Angeles, Calif., assignor toCalifornia Institute Research Foundation, Pasadena, Calif., acorporation of California Application July 30, 1956, Serial No. 600,840

16 Claims. (Cl. 315-335) This invention relates to microwave tubes andmore particularly to means for dissipating the heat developed in theslow-wave structure of a traveling-wave tube.

The traveling-wave tube consists, in essence, of an electricallyconducting structure so arranged as to cause electrical signals of veryhigh frequency which are placed upon the structure to propagate alongthe principal axis of the structure at a velocity which is a smallfraction of the velocity of light. An electron stream from an electrongun is simultaneously sent along the axis of the propogating structureat a velocity almost equal to that of the signal propagation. Underthese conditions an interaction occurs between the wave on the structureappropriate to thesignal and the electron stream which is injected atthe velocity of the wave which causes the wave amplitude to increase thesame number of times its starting value in each increment of distance.This type of growth is commonly referred to as an exponential increasein amplitude with distance.

Unfortunately one of the limitations in operating such a structure ashas just been described, is that the electron stream cannot be shotthrough an appropriate long circuit without at least some fraction ofthe electrons striking the circuit. A limitation on the power which canbe amplified in this way is the power which can be sent into theelectron stream without causing the metallic circuit to become so hotthat it gives off gas or vaporizes and makes the tube inoperative. Ofcourse, since an electron stream is used, the circuit and stream areusually placed in a vacuum container and the whole device operated athigh vacuum.

An object of the present invention is to provide for a high degree ofcooling of the helical wire which forms the circuit on which the wavespropagate at low velocity. The present invention provides for theremoval of heat from each turn of the principal helix or helical wire byattaching to each turn a metallic connection from this turn to anoutside metallic pipe which is more easily cooled. This outside metallicpipe is the actual vacuum envelope and hence on its outside surface isin direct contact with cooling water or cooling air. In other cases thelarge outside metallic surface may be just inside of the vacuum envelopebut may be so large that both by conduction through a glass wall, forexample, and by radiation of heat at a relatively low temperature, itremains at a sufliciently low temperature that no vaporization ordegassing occurs.

In order to prevent the metallic connection from the propagating helicalstructure to the outside metallic wall from disturbing the wavespropagating on the structure, it has proven useful to make theconnecting member onequarter of a wavelength long at the middle of thefrequency band which it is intended should be amplified. The middle ofthe band is specified because, since the wavelength of the waves changesas one changes the frequency, it is impossible that the supportingmember or supporting stub be exactly one-quarter of a wavelength longfor a'wide range of frequencies or wavelengths. If

2,353,64i4 Patented Sept. 23, 1958 ICC it is chosen to be exactly rightin the middle of the operating band it will be nearly the right lengthfor adjacent wavelengths not too far removed from the mid-bandwavelengths. The length of one-quarter wavelength is used because atthis length no effect upon the propagating waves of the main helixoccurs. Such a one-quarter wavelength stub supported helix has been thesubject of a previous patent application, notably that of L. M. Fieldand C. K- Birdsall, Serial No. 490,088, filed February 23, 1955; andSerial No. 503,590, filed April 25, 1955.

The present invention describes means and techniques for making suchstub supports such as required for tubes to work at a few thousand voltsat frequencies of 10,000 me. and higher. It utilizes instead of a simpleradial stub, a pair of curved semi-circular stubs leading over to theside wall. Such stubs are produced with the great accuracy of length andspacing required by making them as a wound helix and then simultaneouslybrazing them to the side wall and to the helix which they are tosupport. By winding the supporing helix to precisely the same pitch asthe actual propagating helix and not re moving either one from themandril on which it is wound until after they are brazed together, apair of supporting stubs connecting to each turn of the propagatinghelix is produced with extreme accuracy and ease. At the same time, thesupporting helix is brazed to a ridge along the side wall. This ridgeshorts the supporting helix from turn to turn in such a way that it nolonger acts as a helix on its own, but rather as simply a series ofquarter wave stub supports to the main helix. One-half of acircumference of the supporting helix is almost exactly a quarterwavelength of the free space wavelength for the frequency at mid-band.

An object of the invention is therefore to provide im proved means forconducting heat from a traveling-wave tube slow-wave structure withoutproducing a substan tial' change in the impedance of that structure overthe operating frequency band of the tube.

Another object of the present invention is to provide improved means forsupporting the active helix slow wave transmission line of helixtraveling wave tubes.

Another object of the present invention is to provide a helix supportingstructure for very small diameter helices such as are operated in veryhigh frequency traveling-wave amplifiers, which supporting structure isarranged to provide good heat conduction from the helix, so thatrelatively high beam current and power characterize the electron beamwith which the radio frequency currents in the helix are to interact.

Another object of the present invention is to provide a helix supportingstructure which is fabricated by the use of conventional precision helixwinding processes.

Another object of the present invention is to provide a supported helixstructure suitable for wide band, continuous wave operation in a veryhigh frequency traveling wave-tube.

The novel features which are believed to be characteristic of theinvention are set forth with particularity in the appended claims. Theinvention itself, however, both as to its organization and method ofoperation, together with additional objects and advantages of theinvention, may be better understood when taken in connection with theaccompanying drawings in which several embodiments of the invention areillustrated by way of example. It is to be expressly understood,however, that the drawings are for the purpose of illustration anddescription only, and are not intended as a definition of the limits ofthe invention.

Fig. 1 is a sectional view of a traveling-wave amplifier embodying thepresent invention;

Figs. 3, 4, 5 and 6 are views of three different alternative embodimentsof the heat conducting and slow-wave structure of the amplifier of Fig.1.

Referring to Fig. 1, a traveling-wave-tube amplifier 10 is illustratedhaving a cylindrical, conductive, non-magnetic envelope 12 which may bemade of copper. An electron gun 14 is sealed in the leftextrernity'ofthe envelope, as viewed in Fig. l. Electrorr'gun 14 is employed toproduce a stream of electrons and to direct it along the longitudinalaxis of envelope 12.

A solenoid 16 is disposed concentrically about envelope 12 to provide anaxial magnetic field along the electron stream path whereby the streammay be constrained along the complete length of the envelope. Such afield may be of the order of 600 to 1200 gauss. In order to produce themagnetic field, a direct current is maintained in sole noid 16 by meansof a potential source 18.

A non-magnetic conductive'cooling tank 19 having'the shape of adouble-walled hollow cylinder and which may also be made of copper, isdisposed concentrically between solenoid 16 and envelope 12. A liquid orgaseous coolant may thus be circulated through the tank to conduct heataway from both solenoid 16 and envelope 12.

Electron gun 14 essentially comprises a cathode cylinder 20, a heater22, a focusing electrode 24, and an accelerating anode 26. Heater 22 isconnected across a suitable source of potential 28, the negative side ofthe heater being connected to cathode 20. The negative side of source 28is then connected to the negative terminal of a potential source 30, thepositive side of which is connected to ground in order to maintaincathode at a potential of about 30,000 to 35,000 volts negative withrespect to ground. Focusing electrode 24 has a frusto-conical shape withan internal surface of revolution forming an angle of 67 /2 degrees withits axis of symmetry. Focusing electrode 24 is maintained at the samepotential as cathode 20 by an appropriate connection thereto.Accelerating anode 26 is maintained at about 200 volts positive withrespect to ground by a connection to a tap 32 on potential source 30.

A disc-shaped'magnetic pole piece 33 and dielectric spacer 34 aredisposed contiguously to a cylindrical appendage 27 of anode 26 tomaintain the gun 14 in alignment with envelope 12. A glass stem 36 withintegral leads 37 Welded to the gun 14 and an anode lead 38 comprise theremaining supports of gun 14.

Within envelope 12, adjacent to and to the right of gun 14, as viewed inFig. 1, slow-wave structure 40 in the form of a helix is provided inelectrical contact with and supported in part by a rectangular internalinput waveguide segment 42 and a rectangular internal output waveguidesegment 44. The slow-wave structure 40 and Waveguide segments 42 and 44,which may consist of copl per, are all maintained at ground potential byan electrical connection 45 from envelope 12 to ground. Waveguidesegments 42 and 44 are positioned in contact with envelope 12 and sleeve35. This will be better understood as hereinafter explained inconnection with Figure 2.

Slow-wave structure 40'comprises a helical wire. Waveguide segments 42and 44 have transverse end portions 43 and 49, respectively, whichhave'fa'cingapertures 53 and vided at their outer ends with mica windows62and 63 respectively, which provide vacuumseals. A vacuum is thusmaintained from the windows 62 and 63 to the opposite end of envelope 12at'the'extreme left of glass stem 36.

The stream electrons are intercepted by a collector electrode 52 at theopposite extremity of envelope 12 with respect to electron gun 14.Collector 52, which protrudes outside of the envelope 12, is alsosupported by a suitable aperture in dielectric spacer 54 so as to have alarge surface external to the evacuated chamber for heat dissipationpurposes and may include fins as shown to aid in conducting away theheat that is generated by the stream electrons when they are collected.Accordingly, collector 52 is preferably fabricated of a metal havinggood electrical and heat conducting properties, as in the case ofstructure 40, such as, for example, copper. A potential of the order of200 volts positive with respect to structure 40 is applied to collector52 in order to prevent secondary electrons which may be produced by thestream electrons impinging upon its surface from reaching slow-wavestructure 40. This potential is applied by means of a connection fromcollector 52 to the positive terminal of a source 56, the negativeterminal of which is grounded.

The helical wire 40 is supported in a novel manner, using anotherhelical wire 51. These two Wires have the same pitch. The supportinghelix 51 is brazed to the active helix 4%) at every single turn, and ata point diametrically opposite to where it touches the helix, it isbrazed to the cooling fin or pedestal 35A extending from the envelope orsleeve 35. Each turn of the active helix then has two quarter wavelength long metallic connections over to the cooling fin as indicated inFigure 2, the two greater wave length sections of each supporting coilturn being designated by reference numerals 51A, 5113. Each of thesections 51A, 513 extends in an arcuate path from the slow-wavestructure 40 a distance equal to where n is any positive odd integer andA is the free space wave-length corresponding to mid-frequency of theoperating band for which the tube 10 is designed. This requirement ismade in order to avoid changing the impedance or" the slow-wavestructure in the center of the operating band, and very little about thecenter. Wave propagation at the band center will thus be essentially thesame with or without the use of the sections 51A,

In the modified arrangement as shown in Figure 3, the active helix 140,support helix 51 and the grounded envelope 35 are inserted as a unitinto the tubular glass vacuum envelope Coupling of high frequency energyin this case is obtained by inserting the ends of the active helix intoa wave guide structure 92. The ends of the helix 140 which define theinput and output ends of the active helix serve as probes 140A that aresealed in the glass envelope 90.

In the modified arrangement shown in Figure 4, a ceramic rod 92 ispositioned within the supporting helix 51. While the supporting helix 51tends to support no radio frequency fields of the normal helix mode,nonetheless when the active helix 40 is exited, strong radio frequencyfields of a different type may exist in certain portions of thesupporting helix 51. It is thus possible to provide the requiredlocalized attenuation while supporting within one or more of thesupporting helices a ceramic rod 92 or other element with a carboncoating or other impregnation. The rod may be treated also with otherlossy materials.

Figure 5 illustrates another alternative arrangement in which threesupporting helices have corresponding turns raised on the one end to.the active helix 140 and the metallic envelope 12 in the manner asillustrated in connection with Figures 1 and 2. 1

Inasmuch as no current flows onto these supporting arcuate one-quarterwavelength stubs 51A, 51B at the frequency at which they are'exactlyone-quarter wavelength long (provided they are perfect conductorsv ordecrease rapidly in amplitude as they travel.

lossle'ss material it is possible to provide not only a V I small effectupon the propagation of the waves for frequencies which differ from thismid-band frequency but also it is possible to make the structure showappreciable loss or attenuation to waves propagating on it atfrequencies to either side of the center frequency. This is done bymaking the stubs somewhat lossy, as indicated above, or by making themhave a moderately high surface resistivity. Then, at frequencies off thecenter frequency where the stubs are not one-quarter wavelength long,appreciable current flows into the stubs and they have moderately highsurface resistivity, as taught in accordance with other aspects of thepresent invention, they cause any waves propagating on the structure toThis is a very useful characteristic inasmuch as such attenuation maynot be necessary at or near mid-band to prevent oscillation (becausenear mid-band the circuit ends may be extremely well matched to externalcircuits and hence no reflections at these ends will occur of sufiicientmagnirude to cause self-oscillation), but it may be very necessary wellaway from this mid-band frequency where the matches tend to become badlyreflected and hence start to produce oscillation, unless the circuitshows appreciable attenuation. The matches can, in general, only be madeextremely good over a limited frequency range,

:and this limiting range could then be placed around the :mid-bandfrequency. Since it is generally true that a 'circuit made lossy toradio frequencies by making it have high resistance also becomes a poorheat conductor, one :might first consider that the heat removalproperties are seriously impaired by making the circuit somewhat lossy.This, however, need not be the case when the supporting stubs are poorheat and electrical conductors, but are covered with a thin film whichhas a high surface resistivity. When this film is sufficiently thick, inthe order of one-thousandths of an inch for 10,000 megacycles, almostall of the radio frequency current at this frequency fiows in thesurface skin and hence the material appears quite lossy. At the sametime, the heat is conducted throughout the closed section of thesupporting wires and the majority of it is conducted by the high heatconducting inner portions of the stubs.

It is thus seen that according to the present invention a new method ofsupporting helix propagation lines is devise-d which is relatively easyto use in manufacturing and which produces a support in helix havingexcellent microwave properties. While the stub supported structuredescribed and claimed in the aforementioned co-pending application isuseful for relatively high power, long wavelength helix tubes,difiiculties are encountered to such an extent to render the stubs showntherein inappropriate as a support for a small diameter, fine pitchhelix as used for instance at X band frequencies in a traveling-wavetube designed to produce ten or twenty watts of continuous wave power.The mechanical problem of attaching these quarter-wave long stubs to thehelix then becomes difficult, as the successive turns are very closetogether. A typical example of such a structure is one requiring a helixhaving sixty turns per inch in which the stubs would then be spaced .002inch apart. Such a stub supported structure would evidently be verydifficult to make it exactly uniform; and the requirements foruniformity makes the design practically impossible to realize in auseful tube. At frequencies in the K band, or at even shorterwavelengths, the situation becomes correspondingly worse.

Thus, according to the present invention, a new method of supportinghelix propagation lines is devised which is relatively easy to use inmanufacturing and which produces a supported helix having excellentmicrowave properties, essentially the same as those of the stubsupported helix.

In the manufacturing process, a first helix is wound using the normalhelix winding technique, which helix is to serve as the slow wavepropagating helix and through which the electron beam is projected. Thefirst helix is wound on an oxidized stainless steel mandrel and is thenretained'on the mandrel. A second helix, usually of different diameter,is then wound on a second similar mandrel with precisely the same pitchas the first one and is retained on its mandrel. The ratios of thediameter of the two helices may be in the order of two or three to one,or thereabouts, the diameter of the active helix being determined by thedesired properties of it as a wave propagation element, and the diameterof the second helix being determined by the need to make each half turnof a helix effectively one-quarter wave in length.

.The two mandrels are then aligned so that their axes are parallel andare placed in a jig so that the first turn of the first helix touchesthe first turn of the second helix and so that each successive turn ofthe first helix touches a succeeding turn of the second helix. Thestructure is held in this manner in the jig and is placed in a platingsolution so that a copper or gold or other plating may be applied toboth helices. After plating and appropriate cleaning, the structure,while still being held in position by the jig, is placed in a brazingfinish, and the contacts between the turns of the helices are brazed.The mandrels, after the structure has cooled, are moved directly, or ifthey cannot be removed from the helices easily, are slightly etchedchemically and are then removed from the helices. If properly oxidizedor otherwise coated, the mandrels slip out of the coils easily.

After brazing the support helix to the active helix,

' portions of the outside of the supporting helix may be coated with anywell known carbon or metallic material which is lossy in the frequencyrange being used. If desired, simple resistance wire may be used informing the support coil but it is considered that the thermo lossproblem limits the use of such a structure to a traveling-wave tube of aparticular design.

By the use of any of the embodiments of the device of the presentinvention stop bands may be provided or avoided as desired and heat maybe conducted from traveling-wave tube slow-wave structures withoutreducing their forward wave impedances or their impedances to Waveswithin their respective operating frequency bands.-

What is claimed is: v

1. In a wave-type amplifier having a conductive slowwave structure-inthe form of a helical coil for propagating electromagnetic waves, ahelical supporting coil, means providing axially periodicdiscontinuities along the slow-Wave structure comprising consecutivehalf turns of said supporting coil extending radially from and beingconnected to the slow-wave structure at predetermined intervalstherealong, and conductive means connecting the extremities of said halfturns remote from said slowwave structure conductors.

2. The invention as defined in claim 2, wherein said conductive meansare spaced from the slow-wave structure, where n is any positive oddinteger, and A is the free space wavelength of the mid-frequency of theoperating band of said amplifier.

3. The invention as defined in claim 2, wherein said predeterminedintervals are equal to where m is any positive integer, and a is theunloaded waveguide wavelength of the mid-frequency of the backward waveself-oscillation band of the slow-wave struc ture.

4. A wave-type amplifier comprising an evacuated envelope, an electrongun disposed at one end of said envelope for producing an electronstream, a collector electrode disposed 'atlthe opposite'end of saidenvelope to in tercept the stream -e1e'ctrons,'means for directing saidstream along a predetermined path from-said electron gun to saidcollector electrode, a slow-Wave structure disposed contiguously aboutsaid path forpropagating electromagnetic waves, 'and a helicalsupporting coil having a plurality of consecutive arcuate conductorshaving a thickness small in comparison to the circumference of saidslow-Wave structure, and conductive means connecting the outerextremities of said conductors to conduct heat away from said slow-wavestructure.

5. A wave-type amplifier comprising a conductive slow-wave structure forpropagating electromagnetic Waves within a predetermined frequency bandhaving a mid-frequency corresponding to a free space wavelength of ksaid slow-wave structure including a coil disposed concentrically aboutsaid path,a helical supporting coil having a plurality of consecutiveconductive arcuate fins having an arcuate thickness small in comparisonto the circumference of said slow-wave structure, each of saidconductive fins extending arcuately away from a convolution on said coila distance substantially equal to I where n is a positive odd integer,and a conductive cylinder disposed about said conductive fins, the outerextremities of said conductive fins being in physical and electricalcontact with said conductive cylinder, whereby heat may be'conductedaway from said slow-wave structure without materially reducing theimpedance of said slow-wave structure to waves of frequencies withinsaid predetermined band.

6. A wave-type amplifier comprising an evacuated envelope; an electrongun disposed at one end of said envelope for producing an electronstream; a collector electrode disposed at the opposite "end of saidenvelope to intercept the stream electrons; means for directing saidstream along a predetermined path from said electron gun to saidcollector electrode; a slow-wave structure disposed contiguously aboutsaid path for propagating waves Within a predetermined frequency bandhaving a mid-frequency corresponding to a free space Wavelength of Asaid slow-wave structure comprising a coil, concentrically about saidpath, a supporting coil of the same pitch as the first mentioned coilhaving convolutions thereof on contact with corresponding convolutionson the first mentioned coil, each of said convolutions of saidsupporting coil comprising a pair of arcuate conductive fins, saidconductive fins extending arcuately from said first coil a distancesubstantially equal to where n is any positive odd integer; andconductive means connecting the outer extremities of said conductivefins, whereby heat may be conducted away from said slowwave structurewithout materially reducing the impedance of said slow-wave structure towaves of frequencies within said predetermined band.

7. In a wave-type amplifier having a conductive helix for propagatingelectromagnetic waves, means providing axially periodic discontinuitiesalong the conductive helix comprising a supporting coil having the samepitchas said helix-withadjacent convolutions of said helix and coilbeing connected.

8. In a wave-type amplifier having a conductive helix for propagatingelectromagnetic waves, means comprising a supporting coil havingconvolutions thereof, connected to'the helix at predetermined intervalstherealong, and conductive means interconnecting the convolutions ofsaid supporting coil at regions spaced substantially from the conductivehelix-where n is any positive odd integer and A is the freespacewavelength ofthe midfrequency of the operating band of the amplifier,said predetermined intervalsbeing equal to helix for propagatingelectromagnetic waves; a heat dissipating envelope surrounding'saidactive helix and extending longitudinally thereof; and at least onesupporting helix having convolutions joined to said active helix and tosaid envelope, said supporting helix being formed of heat conductivematerial to transfer heat through said convolutions from said activehelix to said envelope.

12. A Wave-type-amplifier as set forth in claim 11 wherein: a pluralityof equally spaced supporting helices are disposed between'said activehelix and said envelope.

13. A wave-type amplifier as set forth in claim 11 wherein: a rod havinga surface of lossy material extends through said supporting coil toeffect localized attenuation.

14. A wave-type amplifier as set forth in claim 11 wherein: saidsupporting coil is coated with a lossy material.

15. A wave-type amplifier as set forth in claim 11 wherein: the ends ofsaid active helix define radially directed probes extending through andinsulated from said envelope; and a Wave guide receives each of saidprobes.

16. A wave-type amplifier as set forth in claim 11 wherein: Wave-guidesreceive the axial ends of said active helix; and an electron gunincluding a focusing electrode and accelerating anode is disposedcoaxially of said active helix at'one end thereof.

References (Iitetl in' the file of this patent UNITED STATES PATENTS2,757,310 Robinson et al July 31, 1956 2,768,322 Fletcher Oct. 23, 1956

